1. Field of the Invention
The present invention generally relates to techniques and systems for providing compensation of phase distortions for a transmitted beam and more specifically, to a multiple aperture phased array (PHASAR) phase compensation technique and apparatus.
2. Description of Prior Art
The minimization or elimination of time-varying distortions is crucial in the design of sensor systems, especially coherent (active laser) systems. Intensity and phase are the two quantities that can be controlled in active laser systems and these quantities are considered with respect to performance of such systems. Since phase distributions within an optical system have a substantially greater impact on performance than intensity, techniques and apparatus for providing compensation of phase distortions have been of interest in the prior art. Phase distortions are induced on laser wave fronts by commonly encountered phenomena such as atmospheric disturbance or inherent optical limitations.
Optical limitations and atmospheric effects affect sensor performance by causing random phase distortions whereby there is a reduction in a transmitted beam's field transverse coherence length which causes the beam divergence to be greater than the diffraction-limited divergence. For NIR, the transverse field coherence length can be as small as a few millimeters for ranges of a few kilometers on horizontal paths in moderate to strong turbulence. Thus, the aperture spacing for a multiple aperture phase compensation system should be on the order of one millimeter. Multiple aperture systems using AO/EO phase shifters and conventional lenses have difficulty in meeting this requirement on aperture spacing. In addition, conventional optics implementation would be complex, fragile, and subject to misalignment problems.
Coherent optical adaptive techniques (COAT) have been applied as a technique to minimize phase distortions from atmospheric disturbances. A COAT system generally tries to determine the phase distortions automatically in real time and apply a time-dependent "predistortion" to the transmitted beam wave front so that the net result after propagation through the atmosphere is a diffraction-limited wavefront, including time-varying distortions. COAT systems can be classified as outgoing-wave and return-wave systems. In an outgoing wave system the energy on a target is measured directly by a detector in the target plane, or indirectly by a detector in the transmitter plane receiving the scattered laser radiation from a target. A servo-loop changes the wavefronts of the outgoing wave to maximize the energy on the target. In a return-wave system the phase distortions are measured at the transceiver using: target glint returns, or a beacon laser on the target for heterodyne phase conjugate systems, or scattered laser radiation, or scattered broadband radiation, or target self-emission for compensated imaging systems. COAT systems are at a disadvantage since the target is required to be cooperative, i.e. have a glint, beacon laser,or some other type of coherent energy emission.
In 1977 C. L. Hayes et al disclosed an experimental test in the Journal of the Optical Society of America, Volume 67, No. 3, pages 269-277, to a type of COAT system called an infrared phase conjugated adaptive array. This system not only compensated for phase distortions on the outgoing wavefront to maximize energy on a target, but also compensated for phase distortions on the return wavefront. In their system there are two feedback loops for phase control. One loop controlled the outgoing wavefront's phase to insure that the phase conjugate of the received wavefront was transmitted. The other loop adjusted the phase of the received signal in each channel so that they were equal to each other. This allowed the coherent summation of the received signals from each channel. This system also suffers from the same disadvantage as other COAT systems noted above.
While the prior art has reported using techniques and apparatus to minimize phase distortions none have established a basis for a specific technique and apparatus that is dedicated to the task of resolving the particular problem at hand.
What is needed in this instance is a technique and apparatus that will substantially compensate for temporal distortions, such as due to optical aberrations, atmospheric distortions, and speckle for transmitted signals for an extended target with and without a glint point or beacon laser.